Payload tx table final entry should be implicit

See https://github.com/EspressoSystems/espresso-sequencer/pull/756#discussion_r1394549468 .

Currently the end of the final tx is listed explicitly in the tx table. But this position should instead be implicitly defined as the end of the namespace/payload, which can be found in the namespace table. Thus, for `n` txs the tx table will have only `n-1` entries. That's good because it saves space but now there's a pesky edge case to deal with.